Ammonia synthesis is one of the fundamental processes in the chemical industries, and the Haber-Bosch method using iron oxide as a catalyst and potassium hydroxide as a promoter is widely used. This method has not largely changed in about 100 years. In ammonia synthesis by the Haber-Bosch method, the synthesis is performed by reacting nitrogen gas and hydrogen gas on a catalyst under high-temperature and high-pressure conditions at 300° C. to 500° C. and 20 to 40 MPa. The reaction for synthesizing ammonia using a gas containing hydrogen and nitrogen as raw materials is represented by N2+3H22NH3, however, this reaction is an exothermic reaction, and therefore, in order to shift the equilibrium to the right, a lower temperature is better. However, the number of molecules is decreased by the reaction, and therefore, in order to shift the equilibrium to the right, a higher pressure is better.
However, a nitrogen molecule has a very strong triple bond between nitrogen atoms, and therefore has extremely poor reactivity and the reaction between nitrogen and hydrogen is extremely slow. Therefore, it was extremely important to develop a catalyst capable of activating a nitrogen molecule by breaking the triple bond of the nitrogen molecule. Haber et al. used an iron ore as a catalyst. This iron ore contains iron oxide as a main component and also contains alumina and potassium oxide. In the Haber-Bosch method, iron oxide is packed in a reactor as a catalyst, however, what actually reacts is metallic iron generated by reduction with hydrogen. Alumina works as a support without being reduced and prevents iron particles from sintering, and potassium oxide donates electrons to iron particles as a base to enhance the catalytic ability. Due to these actions, it is called “doubly promoted iron catalyst”. However, even if this iron catalyst is used, the reaction rate is not sufficient at a low temperature of 400° C. or lower.
In a conventional industrial technique, hydrogen is produced by reforming natural gas or the like and is reacted with nitrogen in the air under the conditions above in the same plant, whereby ammonia is synthesized. As the catalyst for ammonia synthesis, conventionally, Fe/Fe3O4 is mainly used, however, recently, an Fe/C or Ru/C catalyst in which active carbon is used as a support is also used.
It is known that when Ru is formed on a support as metal catalyst particles for ammonia synthesis and used, the reaction proceeds at a low pressure, and it has attracted attention as a second generation ammonia synthesis catalyst. However, Ru as a single substance has very small catalytic ability, and in order to make it exhibit an ability to break the triple bond of a nitrogen molecule to convert the molecule to adsorbed nitrogen atoms on the Ru metal catalyst particle, it is preferred to simultaneously use a material having high electron-donating properties, and therefore, it is necessary to use a support composed of a basic material in place of Fe3O4 or active carbon, or to use a promoter compound such as an alkali metal, an alkali metal compound, or an alkaline earth metal compound.
As a catalyst for ammonia synthesis, there are a catalyst, which contains, as a transition metal having an ammonia synthesis activity at a low temperature of 300° C. or lower, one element selected from Mo, W, Re, Fe, Co, Ru, and Os, or at least one combination of Fe and Ru, Ru and Re, and Fe and Mo; K or Na; and alumina, thoria, zirconia, or silica, in which the transition metal and the alkali metal are substantially in a metal state (PTL 1), a catalyst capable of synthesizing ammonia even at a low temperature such as 200° C., in which any of transition metals of Groups 8 and 9 such as Fe, Ru, Os and Co, and an alkali metal are supported on active carbon or porous carbon (PTL 2), a catalyst, in which an alkali metal salt is used in place of an alkali metal, and graphite-containing carbon having a specific surface area is used as a catalyst support (PTL 3), a catalyst for producing ammonia, in which metallic ruthenium or a chlorine-free ruthenium compound and a rare earth element compound, are supported on a hardly reducible oxide such as alumina or magnesia (PTL 4), an ammonia synthesis catalyst, which is composed of Ru, Ni, and Ce, in which at least part of the cerium atoms are in a trivalent state (PTL 5), and the like.
An electride-based catalyst composed of Ru/CaO—Al2O3 incorporates an electron in a crystal structure of a CaO—Al2O3 compound, and can also incorporate a hydrogen atom from surrounding glass during a reaction. It has been reported that from these two characteristics, it has a high catalytic activity for ammonia synthesis (NPL 1), and a patent application was made for an invention relating to an ammonia synthesis method in which nitrogen and hydrogen as raw materials are reacted on a catalyst under conditions of a reaction temperature from 100° C. to 600° C. or lower and a reaction pressure of 10 kPa to 30 MPa (PTL 6).
It has been attempted to apply perovskite composite oxides to various applications such as exhaust gas purification catalysts, superconductive oxides, piezoelectric bodies, sensors, and fuel cell electrolytes. It has been reported that a Ru catalyst supported on a BaCeO3 nanocrystal among the perovskite composite oxides has an excellent catalytic activity at a low temperature of 623 K or lower as an ammonia synthesis catalyst as compared with Ru/γ-Al2O3, Ru/MgO, and Ru/CeO2 catalysts (NPL 2). In addition, an ammonia synthesis catalyst composed of Ru/BaZrO3 (NPL 3, PTL 7, and PTL 8) and an ammonia synthesis catalyst composed of a titanium-containing perovskite oxide such as Ru/BaTiO3, Ru/SrTiO3, or Ru/CaTiO3 have also been reported (NPL 4 and NPL 5), and a patent application has been made (PTL 9).
Titanium-containing oxides having a perovskite crystal structure or a layered perovskite crystal structure represented by MTiO3 (wherein M represents Ca, Ba, Mg, Sr, or Pb), titanium-containing oxides in which some of the Ti atoms are substituted with at least one of Hf and Zr (collectively referred to as “titanium-containing perovskite oxides”) have an extremely high relative dielectric constant, and therefore have been actively studied for a long time as devices such as capacitor materials and dielectric films and also in terms of applications to substrate materials of other perovskite transition metal oxides and nonlinear resistors.
The present inventors have reported the synthesis of titanium oxyhydrides (titanate oxyhydrides) based on the formula: ATi(O,H)3 (wherein A represents Ca2+, Sr2+, or Ba2+) (NPL 6 to NPL 8, and PTL 10). This oxyhydride is a compound in which hydrogen is made to coexist as hydride (H−) with an oxide ion (O2−), and is prepared by a method for reducing a precursor ATiO3 into a topochemical with a metal hydride such as CaH2, LiH, or NaH. This oxyhydride is characterized by having hydride ion-electron mixed conductivity, and hydrogen storage and release properties.